18mpg - I'm stumped

badSentra · Jul 9th, 2004, 07:07 PM

Hi y'all, great thing you got going here. I recently bought an '87 sentra 4wd wagon (e16i with throttle-body injection) and it runs pretty good except the gas milage is awful - 160mi on a tank is the best i've seen. It's pissing me off because I bought the car for good milage. Anyway (with the help of my mechanic) I've checked all the things that seem obvious:
*new air filter
*new fuel filter
*new O2 sensor
*no error codes on computer - all the computer says is that it's running rich and not going into closed loop
*fuel pressure was slightly high
*fuel return line ok
*carbon vapor canister ok
*no backpressure from catalytic converter
*coolant temp sensor within spec
*replaced bad vacuum lines
*MAF ok
*TPS ok
*spark plugs ok but sooty
*wires look new
*cap and rotor look new inside and out

I'm stumped - anyone have any ideas?
Thanks, Alan

minute rice sentra · Jul 9th, 2004, 07:35 PM

Make sure the o2 sensor is getting it's signal back to the ECU and see if the sensor's body has full continuity to ground(it should), kinda sounds like a disconnected sensor or something.

badSentra · Jul 9th, 2004, 09:29 PM

thanks for the tip, but how to test that? my haynes manual gives no hints. i guess measure voltage at the sensor and at the ECU to make sure it's the same? i have had problems with the connectors to the ECU not conducting well. anyone know of a way to ensure a good connection?

i forgot to mention that the car starts well cold but not so good hot. theres also some hesitation at high rpms, cold or hot.
thanks, alan

minute rice sentra · Jul 9th, 2004, 11:08 PM

I wouldn't know about the hard starting problem but for the sensor you've got the Idea, make sure it's not dropping voltage somewhere. You should try to clean up the contacts on the ECU(with maybe rubbing alcohol or something) and put some dielectric grease on them to keep them from corroding.

Popkorn · Jul 10th, 2004, 12:45 AM

Quote:

Originally Posted by minute rice sentra

You should try to clean up the contacts on the ECU(with maybe rubbing alcohol or something) and put some dielectric grease on them to keep them from corroding.

A pencil eraser works wonders on copper contacts. Just be sure to to use the dielectric grease when it's all good.

Gsolo · Jul 10th, 2004, 02:38 AM

18 is pretty bad...i would expect high 20's with a heavy car like that. and you've checked everything i'd thikn of except for the ecu itself.....but usually with ecu's its either it works or it don't. did you or your mechanic ever check the ignition timing though???

mille-16i · Jul 10th, 2004, 02:48 PM

Another posability is that you have a leaky feul injector. also I was having a very similar problem with my 87 sport coupe I relpaced the CTS but still had crappy millage it turned out that my cooling system was not up to par and kept loosing water which in turn did not get the CTS hot. I fixed the cooling system and all was well.

badSentra · Jul 16th, 2004, 12:27 AM

i've finally checked a few more things. i measured voltage at the o2 sensor and at the computer: it was the same (1.1 V when semi-warmed-up). also checked voltage from the coolant temp sensor: same at sensor and computer (can't remember voltage). the connectors all looked clean and shiny but i cleaned them anyway with a spray from napa, and i removed a loose screw that was knocking around inside the computer, but none of these changes improved the situation. so the wiring seems fine, and i think the cooling system/sensor is fine too because resistance at the sensor was consistant with cold engine or hot engine when the engine was cold/hot respectively. i plan to check the timing and compression early next week, but i don't expect either to be far off because the car seems to get decent power (i have very low standards).

does anybody have any other ideas of what the problem could be? are there any guidlines as to what sort of voltage the o2 sensor should be generating?

badSentra · Jul 16th, 2004, 01:22 AM

oh yeah, my mechanic checked for a leaking injector by seeing how well the fuel system retains pressure after the car is shut off. he said it was within spec.

Gsolo · Jul 16th, 2004, 02:11 AM

i didn't think the O2 was suppose to be more than 1 volt

mille-16i · Jul 19th, 2004, 02:36 PM

This is a long shot but sounds like you have touched everything else damn near check for clogged exhaust system if your muffler is clamp on type unhook it and drive a little will be hella loud but might show results. Have you checked for codes from the ecu some times that will help.

RiceBox · Jul 19th, 2004, 03:07 PM

Last ditch effort. Take off the TB and check if your intake manifold is dirty. Try running some fuel system cleaner through. If it's all gummed up with soot and shit, I can see that ruining the mileage.

mille-16i · Jul 19th, 2004, 05:29 PM

just be carefull what you use and to not get alot on rubber orings carb clean and the likes can dry out ruber in a hurry

mille-16i · Jul 19th, 2004, 05:39 PM

Just remembered something from my days in as a mechanic, we were having similar issues lack of power and bad milage with sone fleet trucks. Well we ended up having to hollow out the catalatic converters (not that I am recomending that epa would have my A**) but that may be the culperate cats can glog up fairly easly and they are right off the engine so if all else fails check that with the exhaust system. good luck

Gsolo · Jul 19th, 2004, 06:28 PM

i'm still kinda curious about that O2 sensor.....and yeah cat can clog up...but i get 34.5 mpg even with a clogged cat...i figure if i cut the cat out and do the air cleaner mod from minute rice i could get up to 37mpg.